1. Field of the Invention
The present invention relates to image joining apparatuses capable of joining a plurality of read images together. In particular, the present invention relates to an image joining apparatus capable of reading a document and storing the read images in memory.
2. Description of the Related Art
There have been proposed apparatuses capable of reading a document larger in size than a document table in a plurality of parts, determining relative positions of the plurality of images read, joining the read images together, and recording the resulting image.
For example, Japanese Patent Laid-Open No. 63-142991 discloses a joining method in which, to analyze a correlation between a plurality of images stored in image memory, an image synthesizing processor calculates a mutual correlation coefficient f(τ) to determine positions at which the plurality of images are jointed together.
Similarly, Japanese Patent Laid-Open No. 2006-119730 discloses a method in which a correlation between joints of images to be joined is calculated.
Additionally, Japanese Patent Laid-Open No. 2003-69757 discloses a technique for reducing the amount of image memory used to store a plurality of images. This technique involves a process in which a one-page document is read line by line by a reading unit, the read image data is compressed, and the compressed image data is stored. By repeating this process, the one-page document is stored in image memory. Thus, when image compression is performed every time a part of the document is read, it is not necessary to store an uncompressed image of the one-page document in the image memory. In particular, if lossy compression, such as JPEG compression, which offers a high compression ratio is used, it is possible to further reduce the amount of image memory used.
Hereinafter, an example where a read image is subjected to lossy compression, such as JPEG compression, and stored in memory is referred to as a first known example.
On the other hand, there is a method in which a read image is stored as a RAW image. Here, the RAW image is either an uncompressed image or an image compressed by lossless compression. In the uncompressed image, each dot is expressed in an RGB color space. That is, the uncompressed image, which is obtained by simply converting the read data into the RGB color space, is not degraded. Also, the image compressed by lossless compression is not degraded by image expansion. Therefore, a RAW image is suitable for use in techniques disclosed in Japanese Patent Laid-Open No. 63-142991 and Japanese Patent Laid-Open No. 2006-119730 where a correlation between images is calculated.
Hereinafter, an example where a read image is stored as a RAW image in memory will be referred to as a second known example.
In the first known example, where a read image is stored in lossy compressed format, the amount of image memory used can be reduced. However, unlike the second known example where a read image is stored as a RAW image, the first known example suffers from image degradation caused by lossy compression. In particular, when a high-frequency portion, such as a text portion, is compressed in JPEG format, image degradation called mosquito noise occurs. Moreover, since the degraded image is used to calculate a correlation between images as described in Japanese Patent Laid-Open No. 63-142991, the accuracy of the calculation is lower than that in the second known example.
Therefore, as in the second known example, a read image may be stored as a RAW image in image memory. However, since the RAW image is an uncompressed image or a lossless-compressed image, its data size is larger than that of a lossy-compressed image, such as a JPEG image. That is, although the second known example provides higher accuracy in correlation calculation, the amount of memory necessary to store the image data is larger than that in the first known example. This means that the cost involved in manufacturing the apparatus is higher in the second known example.